Pre-swirl inlet guide vane for compressor

ABSTRACT

Two sets of guide vanes are positioned in the combustion air flow path in the inlet duct of a rotary compressor of a gas turbine engine for controllably varying the air mass flow rate according to turbine load conditions. The upstream set which provides a fixed, initial degree of swirl relative to compressor rotational direction and axis and the controllably moveable downstream set which provides a final degree of swirl cooperate to provide controllable swirl over the range of about 0° to 32° in the inlet air incident upon the turbine blades. The two guide vane sets are separated by a distance sufficient to allow turbulence induced by the first set to fully decay before the second set is encountered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention involves improvements in apparatus and methods forcontrolling mass flow rate in rotary compressors, particularlycompressors used in recuperated gas turbine engine applications.

2. Description of the Prior Art

The power output of conventional gas turbine engines can be varied bychanging the turbine inlet temperatures, such as by reducing fuel flow.However, it is known that substantial increases in the part loadefficiency can be achieved if the mass flow rate of the combustion airis reduced to maintain high turbine inlet temperatures, particularly inrecuperated gas turbine engine applications. It is also known thatprecise control of the combustion temperature can lead to reductions inthe amount of undesirable hydrocarbon and nitrogen oxide emissions.

Previous attempts to change or control the air mass flow rate inexternally driven compressors, such as compressors driven by an electricmotor or compressor section of gas turbine electrical power generatorunits, involve the use of guide vanes in the compressor inlet that aremoveable to induce swirl to the incoming air to change the angle atwhich the inlet air enters the compressor blades. Most compressors aredesigned to have blade shapes and angles of attack orientation selectedto obtain optimum mass flow rate at rated speed. These design pointconditions generally presuppose inlet air incident at a fixed,predetermined angle relative to the axis of rotation. Inlet air flowincident at angles different from the design value, such as where swirlis introduced or the amount of swirl is changed, causes the mass flowrate through the apparatus to change from the design value.

A problem with conventional swirl-inducing guide vane apparatus used tovary the mass flow rate in compressors is that the maximum degree ofturning or swirl achievable without substantial separation with a singleset of vanes is approximately 15°, while variations in the swirl angleof about 30° may be desirable in certain applications, such ascompressors used in gas turbine engines, in order to achieve highthermal efficiency throughout the entire operating range. Althoughmoveable vanes with turning angles greater than about 15° have beenattempted, these are susceptible to severe separation and consequentlosses. Various attempts have been made to circumvent this problem suchas by the use of two-piece articulated vanes having a fixed leadingportion and a moveable trailing or tail portion. Another proposedsolution utilizes two sets of vanes, a fixed set immediately upstream ofa moveable set to achieve essentially the same function as thearticulated vanes. These solutions are not satisfactory as less than thedesired range of turning can be achieved in practice commensurate withthe requirement for a reasonably low aerodynamic loss.

SUMMARY OF THE INVENTION

It is believed that the deficiencies of the prior art stem in large partfrom the close aerodynamic coupling between the front and rear portionsof the articulated vanes and between the upstream and downstreamseparate sets of guide vanes. While it is believed that the reason forthe deliberately close coupling in the prior art was to take advantageof the known flow profile incident upon the downstream vane portion orvane set, the lack of smooth, undisturbed streamline flow on thedownstream elements can result in unwanted turbulence and prematureboundary layer separation leading to high losses in turning conditions.The losses imposed on the air flow could also be substantial in thenon-turning conditions because of the proximity of the fixed vanes tothe compressor blades. The improvements of the present invention areintended to circumvent these problems.

In accordance with the purpose of the invention, as embodied and broadlydescribed herein, the improvement in rotary apparatus for compressing acompressible fluid of the kind having a plurality of compressor bladesmounted on a rotating hub positioned in a compressor housing, apreferred fluid flow path extending through the housing, the housinghaving a duct portion extending upstream of the compressor bladesrelative to the fluid flow path and determining, in part, the fluid flowpath, comprises means for controllably varying the fluid mass flow ratethrough the compressor including a first set of guide vanes in the ductfor imparting an initial degree of swirl to the fluid entering the ductrelative to the direction of rotation of the compressor hub; and asecond set of guide vanes positioned in the duct upstream of thecompressor blades, the vanes of the second set being moveable abouttheir axes, the second vane set being positioned a distance downstreamof the first vane set along the fluid flow path sufficient to permitsubstantial decay of the turbulence imparted to the flowing fluid by thefirst vane set prior to the swirling fluid reaching the second, moveablevane set, the second moveable vane set for changing the degree of swirlin the fluid to a final degree of swirl corresponding to a desiredcompressor fluid mass flow rate.

Preferably, the first vane set is fixed, and the individual guide vanesin the first vane set are configured and oriented to impart about +10°to +15° of swirl to the incoming fluid relative to the axis anddirection of rotation of the compressor.

It is further preferred that the first vane set and the second moveablevane set cooperate to provide final fluid swirl of from about 0° to +32°at the inlet to the compressor blades, relative to the axis anddirection of rotation of the compressor.

Also in accordance with the present invention, as embodied and broadlydescribed herein, the method of throttling the compressible fluid massflow rate through a rotary compressor of the kind having a plurality ofcompressor blades on a rotating hub and having an inlet region includinga duct determining, in part, the flow path of the incoming fluid to thecompressor blades, comprises the steps of imparting a first, initialdegree of swirl to the incoming fluid relative to the axis and directionof rotation of the compressor; removing turbulence in the swirling fluidinduced by the initial swirl imparting step; and imparting a second,controllably variable degree of swirl to the swirling fluid to changethe degree of swirl in the fluid to a desired value prior to admittingthe fluid to the compressor blades, the aforementioned steps beingaccomplished in the compressor inlet region.

The accompanying drawing, which is incorporated in and constitutes apart of this specification, illustrates one embodiment of the inventionand, together with the description, serves to explain the principles ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of the improvements in the compressorapparatus associated with a single shaft gas turbine engine, whichimprovements constitute the present invention;

FIG 2a is a schematic showing the inlet air flow path through thecompressor section for low power operation of the gas turbine engineshown in FIG. 1; and

FIG. 2b is a schematic showing the inlet air flow path through theimproved compressor of FIG. 1 at high turbine power operations.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will be made in detail to the present preferred embodiment ofthe invention, an example of which is illustrated in the accompanyingdrawing.

Referring to FIG. 1, there is shown generally a single shaft compressor10 used to increase the pressure of a gaseous fluid such as air.Compressor 10 includes a housing 12 surrounding a hub 14 on which aremounted compressor blades 16. Hub 14 is rotatable about axis 18(illustrated schematically in the Figure). Compressor 10 is shown inFIG. 10 to be of the centrifugal type, with the entrance 20 to thecompressor blades 16 predominantly in the axial direction in relation tothe axis 18 and with the compressor gas leaving compressor 10 at exit 22substantially in the radial direction. However, the improvements of thepresent invention are not restricted to use with centrifugalcompressors, and the scope of the present invention inludes axialcompressors as well as mixed axial and radial flow compressor devices.

The improvements constituting the present invention enable the pressureratio and the mass flow rate to the compressor 10 to be controlledessentially independent of the compressor rotational speed. This is anespecially important advantage in certain applications such as where thecompressor is driven at essentially constant speed such as by asynchronous device or where, such as shown in FIG. 1, the compressor isused in a recuperated single shaft gas turbine engine application. Thepresent invention also can be utilized to advantage in a two shaftmachine because although some decrease in the gas mass flow rate occurswith the decrease in rotational speed of the gas generator in suchmachines, additional reductions can be achieved using the presentinvention.

Thus, FIG. 1 shows compressor 10 associated with heat exchanger 30,combustor 32 and turbine 34, all illustrated schematically. According towell-known principles, the compressed air emanating from the compressorexit 22 is channeled by appropriate ducting 40 through heat exchanger 30where it is heated prior to admission to combustor 32, such as by theexhaust gases channeled from exit 42 of turbine 34 by ducting 44. Theheated compressed air is then combusted with fuel in combustor 32 andthe combustion gases are conveyed by ducting 36 and admitted to theturbine 34 at turbine inlet 38 for subsequent expansion and extractionof mechanical work. Turbine 34 is shown co-axial with axis 18 ofcompressor 10, but other configurations are available depending upon theparticular application.

It is understood that the aforementioned heat exchanger 30, and ducting36, 40, 44 may be constructed as integral parts of housing 12 and/or theturbine housing (not shown) and are depicted as separate components inFIG. 1 merely for convenience of description. Also, the preheating ofthe compressed air can be accomplished by channeling the compressed airemanating from compressor exit 22 past various structural componentssuch as the combustor 32 housing (not shown) and the turbine housing forcooling these components. Although such an arrangement may not have allthe thermodynamic advantages of a fully recuperated gas turbine engine,other advantages, such as extended lifetimes for structural and rotatingcomponents and better control over hydrocarbon and nitrogen oxide (NOX)emissions, may be achieved. For example, it is desirable to utilize leanburning combustors to control NOX emissions but this requires preciselycontrolling the fuel/air ratio over the entire operating range. It isfar easier to control the fuel/air ratio if the air mass flow rate canbe controlled during part load operation, such as by use of the presentinvention. Thus, the scope of the present invention is intended toinclude these latter applications as well as the recuperated gas turbineengine application depicted in FIG. 1.

In accordance with the present invention, means are provided forcontrollably varying the fluid mass flow rate through the compressor,the means including a set of final, moveable guide vanes positionedupstream of the compressor blades. As embodied herein, compressorhousing 12 has an inlet duct portion 50 extending upstream of compressorblades 16 along the inlet air flow path through the compressor 10(designated by arrows 54). Duct 50 is shown as having a shroud side 50a,a hub side 50b, and an air inlet 66. A set of guide vanes 52 havingleading edges 52a and trailing edges 52b are positioned in compressorinlet duct 50 upstream of compressor blade 16, and the function of vanes52 is to impart a final controllably variable degree of swirl to thefluid in flow path 54, relative to the direction of rotation of hub 14on compressor axis 18, the final degree of swirl corresponding to thedesired compressor mass flow rate. Vanes 52 are attached to duct 50 byblade mounting assembly 56 which provides for rotational movement ofvanes 52 about the respective vane longitudinal axis 58 such as topresent a varying angle of attack to the incident air flow 54. Changesin the angular orientation of vanes 52 about axis 58 are accomplishedthrough the lever assembly 60 attached to vanes 52, but any suitablemechanical, hydraulic or other actuating mechanism can be used.Actuating mechanisms such as lever assembly 60 can be automaticallycontrolled by conventional controller means (not shown) in accordancewith desired operating conditions, or would be immediately evident tothose skilled in the art.

Preferably, the vanes 52 are positioned in duct 50 adjacent thecompressor entrance 20 and proximate compressor blades 16 along flowpath 54 for the following reasons. Vanes 52, however, should be spacedfrom compressor blades 16 a distance sufficient to allow any wakegenerated in the inlet air by vanes 52 to close before the inlet airreaches blades 16. The final flow profile incident upon compressorblades 16 will be determined by vanes 52 as will be discussedhereinafter, and the proximity of vanes 52 to compressor blades 16 willsecure definition of the final swirl profile insofar as there will beminimal interaction with housing portion 50. It is also preferred foressentially the same reasons that vanes 52 be located in a region ofduct 50 wherein the average streamline velocity at the leading edge ofthe vanes is at least about 70% and, more preferably, more than about80% of the average streamline velocity at compressor entrance 20.

In accordance with the present invention, there is further provided aset of initial guide vanes for imparting an intial degree of swirl tothe inlet air entering the compressor, relative to the axis anddirection of rotation of the compressor, thus dividing or sharing thetotal turning between the two separate sets of guide vanes. As embodiedherein, vanes 62 are positioned in duct 50 upstream of movable vanes 52along flow path 54 and near air inlet region 66. Thus, the inlet airentering compressor 10 along flow path 54 at inlet 66 is influencedfirst by guide vanes 62 and then second by guide vanes 52 before beingadmitted to the compressor blades 16.

Preferably, the vanes 62 present to the air flow entering the duct 50 atinlet 66 along flow path 54 an angle of attack that remains constant intime during operation of the compressor 10 over the entire range of loadconditions, although the angle may vary spatially along the vane axis toachieve a desired aerodynamic flow pattern in a particular inlet ductconfiguration. For instance, in the configuration for duct 50 shown inFIG. 1, the velocities near the shroud side 50a will be higher than theinlet air velocities near the hub side 50b. Thus, the portion of thevane 62 near the hub side may have a greater angle of attack than theshoud side portion giving rise to a "twist" in the profile of vanes 62.Vanes 52 can also have a twist to further match the incident flowprofile, that is, to provide a spatially constant angle of attack to theair flow incident from vanes 62. In general, the "twist" in vanes 52will be less than that in vanes 62.

To achieve an angle of attack that is constant in time, the vanes 62 canbe permanently fixed in duct 50 such as by making them an integralhousing structural member or can be attached by suitable fasteningmeans. As shown in FIG. 1, the vanes 62 are fastened by boltingmechanism to permit adjustment changes in the angle of attack of thevanes 62 during initial assembly of the gas turbine apparatus or duringsubsequent servicing outages, in order to achieve optimum results.

Importantly, the distance between initial guide vanes 62 which impart aconstant degree of initial swirl to the inlet air and the final,moveable guide vanes 52 which impart a final degree of swirl to theinlet air, depending upon operating load condition, is such as to permitthe turbulence induced by vanes 62 to become essentially decayed inorder to provide substantially invariant streamline flow across the flowarea of flow path 54 immediately upstream of vanes 52. This positioningenables moveable vanes 52 to be aerodynamically decoupled from the guidevanes 62 to the extent that premature boundary layer separation on vanes52 will not be induced by asymmetrical, undecayed wake from vanes 62.

It is estimated that spacing the trailing edges 62b of vanes 62 from theleading edges 52a of vanes 52 a distance of greater than or equal toabout one chord length of vanes 62 should provide essentially fullydecayed turbulent flow incident upon vanes 52. Preferably, vanes 62should be located in a region of relatively low air flow velocities sothat the full load losses are small and on the order of about less than30% of the average streamline flow velocity at compressor entrance 20and, more preferably, less than about 10%. For reasonable ductconfigurations, such as shown in FIG. 1, this requirement entails aphysical separation distance of at least one chord length of vane 62 andusually two to three chord lengths, thus ensuring fully decayed flow atthe leading edges 52a.

In the centrifugal apparatus disclosed in FIG. 1, vanes 62 are locatedin the inlet region 66 of inlet duct 50. The flow path 54 in inlet 66 ispredominantly in the radial direction. Moveable guide vanes 52 arelocated at the exit of inlet duct 50 near compressor entrance 20 in aregion where the flow path 54 is predominantly axial and where averagestreamline flow velocities on the order of about 250 meters/secondoccur. One skilled in the art could determine without undue analysis orexperimentation an appropriate separation distance for a givencompressor inlet duct configuration given the present disclosure.

It is also preferred that the guide vanes 62 are configured and orientedin inlet duct 50 to impart about +10° to 30 15° of initial swirl to theincoming fluid under all turbine load conditions, the degree of initialswirl being measured immediately upstream of vanes 52 along flow path54, relative to the axis 18, and with the direction of rotation of hub14 establishing the positive direction. With reference to FIGS. 2A and2B, the angle of initial swirl is shown by the angle which is positiveas it is in the direction of rotation of hub 14 (designated by arrows).

It is still further preferred that the moveable vanes 52 are configuredand oriented by a lever assembly 60 to impart a further final degree ofswirl to the inlet air such that the final degree of swirl in the fluidincident upon the compressor blades 16 (represented by the angle inFIGS. 2A and 2B) ranges from about 0° to +32° for the maximum andminimum turbine load conditions, respectively. For the apparatus in FIG.1, guide vanes 52 should be capable of changing the relative directionof the air incident on vanes 52 from vanes 62 from about -20° to +20°depending upon the turbine load.

With specific reference to FIGS. 2A, 2B which depict the invention beingused in the compressor component of a gas turbine engine operating atminimum and maximum load conditions, respectively, it is clear that theorientation of vanes 52 at minimum turbine load is such as to increasethe positive degree of swirl induced by vanes 62 while at maximum loadconditions the orientation of moveable vanes 52 is such as to impartnegative swirl to eliminate or correct the final degree of swirl in thecombustion air to the 0° maximum load design condition for thecompressor 10 shown in FIG. 1. If the particular turbine apparatusincludes a compressor having a design point with a finite, non-zero(positive or negative) degree of swirl, the preferred ranges of thesettings for initial set of vanes 62 and final, moveable vanes 52 wouldbe adjusted accordingly, as would be evident to one skilled in the artupon reading this disclosure.

Because of the aerodynamically decoupled nature of the relationshipbetween vanes 62 and moveable vanes 52, a certain amount of analysisand/or experimentation will be necessary in a particular application todetermine the angular settings and twists of vanes 62 and vanes 52needed to effect the desired final degree of swirl at the entrance tocompressor blades 16, particularly in applications such as shown in FIG.1 wherein the compressor inlet duct 50 contains substantial changes inthe flow path 54 direction between vanes 62 and 52. However, thepenalties in terms of increased design cost are outweighted by theexpected increase in overall efficiency of the turbine unit,particularly at low load conditions where large final swirl angles canbe achieved on stable basis. This increased low load efficiency also isexpected to outweigh the slight degradation in performance at themaximum or design load due to the additional pressure drops incurred ininlet duct 50 to, in effect, first induce swirl and then remove theswirl in the inlet air incident upon compressor 16. Although the use ofmoveable vanes for vanes 62 would overcome this deficiency, and isconsidered within the scope of the present invention, the extracomplexity makes such an embodiment not as preferable as the fixed vane62 embodiment shown and described herein.

It will be apparent to those skilled in the art that variousmodifications and variations could be made in the improved means forcontrolling the mass flow rate in a compressor and the correspondingimproved method for achieving control of the mass flow rate withoutdeparting from the scope or spirit of the invention.

What is claimed is:
 1. In rotary apparatus for compressing acompressible fluid of the kind having a plurality of compressor bladesmounted on a rotating hub positioned in a compressor housing, apreferred fluid flow path extending through the housing, the housinghaving a duct portion extending upstream of the compressor bladesrelative to the fluid flow path .[.and.]. .Iadd., with substantially allthe fluid flowing through the duct being guided to the compressorblades, with the duct .Iaddend.determining, in part, the fluid flowpath, the improvement comprising:means for controllably varying thefluid mass flow rate through the compressor including:(a) a first set ofguide vanes .[.in.]. .Iadd.positioned substantially within ep the ductfor imparting an initial degree of swirl to the fluid entering the ductrelative to the axis and direction of rotation of the compressor hub;and (b) a second set of guide vanes positioned .[.in.]. .Iadd.within.Iaddend.the duct upstream of the compressor blades, the vanes of thesaid second set being moveable about their axes, said second vane setbeing positioned a distance downstream of said first vane set along thepreferred fluid flow path wherein at least about one first vane chordlength separates the trailing edges of said first vane set from theleading edges of said second, moveable vane set, to permit substantialdecay of the turbulence imparted to the flowing fluid by said first vaneset prior to said swirling fluid reaching said second, moveable vaneset, said second, moveable vane set for changing the degree of swirl inthe fluid to a final degree of swirl incident upon the compressor bladescorresponding to a desired compressor fluid mass flow rate.Iadd., saidfirst vane chord length being measured at the widest part of said firstvane, and said separation existing along substantially the entire axiallength of said first and said second vanes.Iaddend..
 2. Apparatus as inclaim 1 wherein the separation distance is about 2-3 lengths.
 3. Inrotary apparatus for compressing a compressible fluid of the kind havinga plurality of compressor blades mounted on a rotating hub positioned ina compressor housing, a preferred fluid flow path extending through thehousing, the housing having a duct portion extending upstream of thecompressor blades relative to the fluid flow path and determining, inpart, the fluid flow path, the improvement comprising:means forcontrollably varying the fluid mass flow rate through the compressorincluding:(a) first set of guide vanes in the duct for imparting aninitial degree of swirl to the fluid entering the duct relative to theaxis and direction of rotation of the compressor hub; and (b) a secondset of guide vanes positioned in the duct upstream of the compressorblades, the vanes of the said second set being moveable about theiraxes, said second vanes set being positioned a distance downstream ofsaid first vane set along the preferred fluid flow path sufficient topermit substantial decay of the turbulence imparted to the flowing fluidby said first vane set prior to said swirling fluid reaching saidsecond, moveable vane set, said second, moveable vane set for changingthe degree of swirl in the fluid to a final degree of swirl incidentupon the compressor blades corresponding to a desired compressor fluidmass flow rate, wherein said first vane set is positioned in a region ofthe duct having average streamline fluid velocities of less than about30% of the average streamline velocities at the compressor blades, andwherein said second, moveable vane set is positioned in a region of theduct having average streamline fluid velocities of at least about 70% ofthe average streamline fluid velocities at the compressor blades. 4.Apparatus as in claim 3 wherein said first vane set is positioned in aregion of the duct having average streamline fluid velocities less thanabout 10% of the average streamline velocities at the compressor blades.5. Apparatus as in claim 3 wherein said second, moveable vane set ispositioned in a region of the duct having average streamline fluidvelocities of greater than about 80% of the average streamline fluidvelocities at the compressor blades.
 6. Apparatus as in claim 1 or 3wherein said second, moveable vane set is positioned upstream of andadjacent to the compressor blades.
 7. Apparatus as in claim 1 or 3wherein the duct has a predominantly radial inlet portion and apredominantly axial exit portion, the fluid flow path undergoing asubstantial angular change in direction through the compressor inlet,and wherein said first vane set is located in said inlet portion andsaid second moveable vane set is located in said exit portion. 8.Apparatus as in claim 1 or 3 wherein the individual guide vanes in saidfirst vane set are configured to impart about +10° to +15° of swirl tothe incoming fluid relative to the axis and direction of rotation of thecompressor hub.
 9. Apparatus as in claim 8 wherein said first vane setand said second, moveable vane set cooperate to provide final fluidswirl of from about 0° to +32° at the inlet to the compressor blades,relative to the axis and direction of rotation of the compressor hub.10. Apparatus as in claim 1 or 3 wherein the individual guide vanes insaid second, moveable vane set are moveable to change direction in thefluid incident thereupon by about -20° to +20°, the direction ofrotation of the compressor hub establishing the positive direction. 11.Apparatus as in claim 1 or 3 wherein said first vane set is essentiallyfixed.
 12. In an air-breathing, variable load recuperated gas turbineengine of the kind having a rotary compressor section for compressingthe air for combustion, a combustor for combusting the compressed airwith fuel to produce combustion gases, a turbine for recoveringmechanical work from the combustion gases, and apparatus for recoveringheat valves from the combustion gases and heating the compressed airprior to combustion using the recovered heat values, the compressorfurther including a plurality of compressor blades mounted on a rotatinghub positioned in a compressor housing, the compressor housing having apreferred air flow path extending therethrough, and also having a ductportion extending upstream of the compressor blades relative to the airflow path, the duct determining, in part, the air flow path, theimprovement comprising:means for controllably varying the air mass flowrate through the compressor and to the combustor to maintain highestpossible turbine section inlet temperatures over varying turbine loadconditions, the air mass flow rate varying means including(a) a firstset of guide vanes in the duct for imparting an initial degree of swirlto the fluid entering the compressor housing inlet relative to the axisand direction of rotation of the compressor hub; and (b) a second set ofguide vanes positioned in the duct upstream of the compressor blades,the vanes of said second set being moveable about their axes, saidsecond vane set being positioned a distance downstream of said firstvane set along the fluid flow path wherein at least about one first vanechord length separates the trailing edges of said first vane set fromthe leading edges of said second, moveable vane set, to permitsubstantial decay of the turbulence imparted to the flowing fluid bysaid first vane set prior to said swirling fluid reaching said second,moveable vane set, said second, moveable vane set for changing thedegree of swirl in the fluid to a final degree of swirl corresponding toa predetermined turbine load condition, wherein said first vane set ispositioned in a region of the duct having average streamline fluidvelocities of less than about 30% of the average streamline velocitiesat the compressor blades, and wherein said second, moveable vane set ispositioned in a region of the duct having average streamline fluidvelocities of at least about 70% of the average streamline fluidvelocities at the compressor blades.
 13. A method of throttling thecompressible fluid mass flow rate through a rotary compressor of thekind having a plurality of compressor blades on a rotating hub andhaving an inlet region including a duct determining, in part, thepreferred flow path of the incoming fluid to the compressor blades.Iadd.with substantially all the fluid flowing in the duct being guidedto the compressor blades, .Iaddend.the method comprising the stepsof:(a) imparting a first, initial degree of swirl to the incoming fluidrelative to the axis and direction of rotation of the compressor; (b)removing turbulence in the swirling fluid induced by said initial swirlimparting step; and (c) imparting a second, controllably variable degreeof swirl to the swirling fluid to change the degeee of swirl in thefluid to a desired final value prior to admitting the fluid to thecompressor blades, steps (a)-(c) being accomplished .[.in.].ep.Iadd.substantially within .Iaddend.the duct,wherein said removing stepincludes performing said first swirl imparting step using a first set ofguide vane at a location along the preferred fluid flow path a distanceof at least one first vane chord length upstream of the location wheresaid second variable swirl imparting step is performed to allow theinduced turbulence to decay naturally.Iadd., said first vane chordlength being measured at the widest part of said first vane, and saiddistance existing along substantially the entire axial length of saidfirst vane relative to said variable swirl step location.Iaddend..
 14. Amethod of throttling the compressible fluid mass flow rate through arotary compressor of the kind having a plurality of compressor blades ona rotating hub and having an inlet region including a duct determining,in part, the preferred flow path of the incoming fluid to the compressorblades, the method comprising the steps of:(a) imparting a first,initial degree of swirl to the incoming fluid relative to the axis anddirection of rotation of the compressor; (b) removing turbulence in theswirling fluid induced by said initial swirl imparting step; and (c)imparting a second, controllably variable degree of swirl to theswirling fluid to change the degree of swirl in the fluid to a desiredfinal value prior to admitting the fluid to the compressor blades, steps(a)-(c) being accomplished in the duct,wherein said second, variableswirl imparting step is carried out in a region of average streamlinefluid velocities of at least about 70% of the average streamline fluidvelocities at the compressor blades, and wherein said first swirlimparting step is carried out in a region of average streamline fluidvelocities of less than about 30% of the average streamline flowvelocities of the compressor blades.
 15. The method as in claims 13 or14 wherein said second, variable swirl imparting step is accomplished byvariable angle guide vane means positioned in the fluid flow path, andwherein said first set of guide vanes is positioned in the duct upstreamof the variable guide vane means.
 16. The method as in claim 13 or 14wherein about +10° to +15° of swirl relative to the axis and directionof rotation are imparted to the incoming fluid in said first swirlimparting step, measured immediately upstream of the location in thefluid flow path where the second, variable swirl imparting step iscarried out.
 17. The method as in claim 13 or 14 wherein said second,variable swirl imparting step changes the direction of the incidentfluid from about -20° to +20°.
 18. The method as in claim 13 or 14wherein said second, variable swirl imparting step together with saidfirst swirl imparting step provide a final swirl of between about 0° to+32° prior to admitting the fluid to the compressor blades.
 19. Themethod as in claim 13 or 14 wherein the duct has a predominately radialinlet portion and a predominately axial exit portion, said first swirlimparting step being carried out in the inlet portion and said second,variable swirl imparting step being carried out in the exit portion. 20.An improved method of throttling inlet air flow rate to the combustor ina recuperated gas turbine engine of the type having a rotary compressorwith a plurality of compressor blades on a rotating hub and with aninlet region including a duct determining, in part, the flow path of theincoming air to the compressor blades, for maintaining a highestpossible turbine inlet temperature over variable load conditions, theimprovement comprising the steps of:(a) imparting a first, initialdegree of swirl to the incoming air relative to the axis and directionof rotation of the compressor; (b) removing turbulence in the incomingair induced by said preliminary swirl imparting step; and (c) impartinga second, variable degree of swirl to the incoming air to change thedegree of swirl in the air to a final desired value corresponding to apredetermined turbine load condition prior to admitting the air to thecompressor blades, said steps (a)-(c) being accomplished in the duct,wherein said removing step includes performing said first swirlimparting step using a first set of guide vane and at a location alongthe preferred fluid flow path a distance of at least one first vanechord length upstream of the location where said second variable swirlimparting step is performed to allow the induced turbulence to decaynaturally, wherein said second, variable swirl imparting step is carriedout in a region of average streamline fluid velocities of at least about70% of the average streamline fluid velocities at the compressor blades,and wherein said first swirl imparting step is carried out in a regionof average streamline fluid velocities of less than about 30% of theaverage streamline flow velocities of the compressor blades.